Many printing systems, such as an offset lithographic printing system, require the use of an aqueous based composition commonly known as dampening fluid or fountain solution. In offset lithography the printing system includes a print plate, and the printing portion of the print plate is ink receptive and fountain solution repellent. Conversely, the blank portion of the print plate is fountain solution receptive and ink repellent. When the print plate contacts rollers wet with ink and rollers wet with fountain solution, the fountain solution coats the blank portion of the plate while the ink coats the image portion of the plate. Wetting the blank portion with fountain solution helps prevent printing ink from adhering to blank portions of the print plate.
Within the printing system, fountain solution is collected in one or more fountain solution reservoirs, commonly called trays. One or more rollers positioned in the tray transfers the fountain solution from the tray to a print plate. A circulation system is used to recirculate fountain solution from a fountain solution outlet of each tray to a sump and back through a fountain solution inlet to each tray of the printing system, and the fountain solution is conditioned for reuse as it is circulated. The most common means of driving the fountain solution from the fountain solution trays to the sump is by gravity feed or gravity-assisted feed, while a pump returns the fountain solution from the sump back to the fountain solution trays of the printing system. Circulation systems are available from Dahlgren USA, Inc. of Carrollton, Tex., Baldwin Dampening Systems, a Baldwin Technology Company, of Naugatuck, Conn. and Royse Manufacturing Company of Dallas, Tex.
It is well known in the printing industry that as fountain solution becomes contaminated with particulates, particularly microparticulates, the print quality deteriorates. Sharp quality print runs depend to a large degree on a relatively contaminant free fountain solution. However, contamination of fountain solution is inevitable. The most common fountain solution contaminants are microparticulates from the printing ink and print paper, as well as the ambient environment. During or after extensive print runs, the fountain solution was typically replaced, due to the buildup of microparticulate contaminants in the fountain solution.
Until recently there has been little incentive to filter fountain solution. In some instances, simple sponge filters or disposable bag filters were used, but these filters function principally to protect the sump pump from large debris. Industry followed the practice that fountain solution, being mostly water with inexpensive additives, could be replaced on a fairly regular basis.
New environmental awareness as well as stringent disposal codes, however, have created the need for a system to filter the recirculating fountain solution so that it is maintained relatively contaminant free. For example, in many localities the spent fountain solution is classified as hazardous waste. The practice of frequently discarding and replacing the solution has therefore become cost prohibitive.
Fountain solution filtration systems currently employed include sponge filters, panel filters (see U.S. Pat. No. 4,671,869) or packed beds (see U.S. Pat. No. 4,608,158). Unfortunately, these systems do not offer the degree of filtration needed to prolong the life of fountain solution from days to several weeks. Furthermore, panel filters and packed beds require special housings and fittings and require significant down time for replacement. The special housings and fittings represent expensive capital costs relative to the profit margins present in printing operations.
There is a need then for a filter system that effectively removes microparticulate contaminants from recirculating fountain solution, preferably a system that can be placed in line with minimal additional hardware or housings. Furthermore, there is a need for a filter system that exhibits a high removal rating for microparticulates, has a long life, and is designed to operate under gravity feed or gravity-assisted feed conditions.